1. Field of the Invention
The present invention relates to a digital broadcast receiver, and more particularly to a matched filter of a digital passband which is variable depending on the recovery result of a carrier wave, and a filtering method and a digital broadcast receiver using the same.
2. Description of the Related Art
A digital broadcast receiving technology currently being developed through diverse media (ground wave, cable, satellite) is oriented toward an integrated system structure for integrated digital broadcasting.
In particular, the transmission system of a digital TV (e.g., HDTV) transmitting data at a high bit rate of more than about 20 Mb/s through band-limited 6 MHz requires a modulation/demodulation method of high band efficiency.
Further, the transmission system of a digital TV further requires a channel equalizer to compensate for non-ideal characteristics of a channel such as a symbol/carrier wave for recovery of a symbol and a carrier, multi-path or a Doppler effect, and a highly efficient channel decryption method for reliable transmission in the presence of jitter.
The integrated multimedia digital broadcast receiver is roughly divided into the following three receiving methods.
1) Ground Wave Receiving Method:
    VSB (vestigial side band; SSB; ATSC: U.S. style)    OFDM (orthogonal frequency division multiplexing; DSB: European style)2) Cable Broadcast Receiving Method:    QAM (quadrature amplitude modulation; DSB)3) Satellite Broadcast Receiving Method:    QPSK (quadrature phase shift keying, DSB)
Depending on the receiving methods described above, different technical elements such as an analog receiving section, signal synchronizing method, channel equalizing method, matched filter, an channel decryption method constitute a technology.
However, it is critical to select sharable functions from the above technical elements to develop an optimal multimedia digital broadcast receiver.
FIG. 1 is a block diagram showing a multimedia digital broadcast receiver in general. Referring to FIG. 1, the multimedia digital broadcast receiver comprises an analog receiving section 101 for receiving and converting a radio frequency (RF) to an intermediate frequency (IF), and a digital demodulation section 102 for digitalizing the IF signal.
The analog receiving section comprises: a tuner 101a for converting and outputting an RF signal of 50–860 MHz to a first IF signal; a surface acoustic wave (SAW) for filtering a signal outputted from the tuner 101a; a first oscillator 101c for generating an oscillating frequency to generate a second IF signal; a mixer 101d for down-converting the signal filtered by the SAW to an oscillating frequency generated from the first oscillator 101c and converting the down-converted oscillating frequency to a second IF signal; an automatic gain control (AGC) amplifying section 101e for compensating for a gain of the second IF signal outputted from the mixer 101d; a second oscillator 101f for generating a sampling frequency; and an analog/digital (A/D) converting section 101g for converting the signal amplified by the ACG amplifying section 101e to a digital signal in accordance with the sampling frequency generated from the second oscillator 101f. 
In the general multimedia digital broadcast receiver constituted as above, the analog receiving section 101 receives and converts the RF signal to the IF signal in output thereof.
To be specific, the tuner 101a in the analog receiving section 101 converts the RF signal of 50–860 MHz to the first IF signal of 44 MHz, and outputs the same to the SAW filter 101b employed for the purpose of removing the adjacent channel signal and jitter signal of the carrier wave from the I, Q baseband digital signals outputted from the complex number multiplying section 204b (see FIG. 2b).
Taking as an example for the digital broadcast signal, the SAW filter 101b removes all the interval from an output of the tuner 101a except the band of 6 MHz where information exists so as to be outputted to the mixer 101d. 
Meanwhile, the first oscillator 101c generates an oscillating frequency for generating the second IF signal. The mixer 101d down-converts the signal filtered by the SAW filter 101b to the oscillating frequency of the first oscillator 101c, and converts the down-converted oscillating frequency to the second IF signal to be outputted to the AGC amplifying section 101e. 
The AGC amplifying section 101e compensates for a gain of the second IF signal outputted from the mixer 101d to be outputted to the A/D converting section 101g. Since the signal passed through the SAW filter 101b is weak, the AGC amplifying section 101e compensates for an output of the SAW filter 101b with a signal gain such that the A/D converting section 101g at the tail end can convert an analog signal to a digital signal.
At this stage, the second oscillator 101f generates a sampling frequency for sampling the second IF signal. The A/D converting section 101g converts the signal amplified by the AGC amplifying section 101e to a digital signal in accordance with the sampling frequency generated from the second oscillator 101f. 
The digital demodulation section 102 demodulates the digital signal converted by the analog receiving section 101, and outputs a resultant signal.
Of the constitutional elements of the multimedia digital broadcast receiver, the analog receiving section 101 has a typical structure regardless of a demodulation method, as shown in FIG. 1.
The digital demodulation section 102 has diverse configurations depending on the method of demodulation (VSB, OFDM, QAM, QPSK, etc.). The symbol recovery section, matched filter, phase separation section, carrier wave recovery section, channel equalizing section and the channel decryption section are the basic technical elements of the diverse kinds of digital demodulators.
The matched filter normally has characteristics of a dual filter structure, i.e., a fixed baseband matched filter and a fixed passband matched filter.
The digital demodulator can be embodied in three structures according to the structural characteristics of the matched filter, as shown in FIGS. 2a to 2c. 
FIG. 2a is a schematic view of an OFDM/QAM/QPSK digital demodulator including a fixed baseband matched filter. FIG. 2b is a schematic view of an 8VSB digital demodulator including a fixed passband matched filter. FIG. 2c is a schematic view of a VSB (OFDM)/QAM/QPSK) digital demodulator including both the fixed passband matched filter and the fixed baseband matched filter.
The OFDM/QAM/QPSK digital demodulator in FIG. 2a comprises first and a second complex number multiplying sections 201a, 202a, a symbol recovery section 203a, a fixed baseband matched filter 204a, a carrier wave recovery section 205a, channel equalizing section 206a and a channel decryption section 207a. The output of the fixed baseband matched filter 204a is fed back to the symbol recovery section 203a, while the output of the carrier wave recovery section 205a is fed back to the first and the second complex number multiplying sections 201a, 202a. 
The 8VSB digital demodulator in FIG. 2b comprises a symbol recovery section 201b, fixed passband matched filter 202b, phase separation section 203b, complex number multiplying section 204b, carrier wave recovery section 205b, channel equalizing section 206b and a channel decryption section 207b. The output of the complex number multiplying section 204b is fed back to the symbol recovery section 201b, while the output of the carrier wave recovery section 205b is fed back to the complex number multiplying section 204b. 
The VSB (OFDM)/QAM/QPSK) digital demodulator in FIG. 2c comprises a symbol recovery section 201c, fixed passband matched filter 202c, phase separation section 203c, complex number multiplying section 204c, fixed baseband matched filter 205c, carrier wave recovery section 206c, channel equalizing section 207c and a channel decryption section 208c. The output of the complex number multiplying section 204c is fed back to the symbol recovery section 201c, while the output of the carrier wave section 206c is fed back to the complex number multiplying section 204c. 
FIGS. 6a to 6c are schematic views illustrating an impulse response and a frequency response of the conventional fixed baseband matched filter and the fixed passband matched filter in 65-tap size of a symmetrical structure. Referring to FIGS. 6a to 6c, the roll-off factor of the fixed baseband matched filter and the fixed passband matched filter is 0.12, while their signal band is 6 MHz. The intermediate frequency of the fixed passband matched filter C is 5 MHz with a sampling frequency of 20 MHz. To be specific, FIG. 6b shows an impulse response of the fixed baseband matched filter and the fixed passband matched filter, in which the horizontal axis represents a frequency, while the vertical axis represents a decibel value.
As the basic technical elements of the digital demodulators are similar to each other as shown in FIGS. 2a to 2c, the 8VSB digital demodulator in FIG. 2b will be exemplified as an embodiment hereinafter.
The symbol recovery section 201b receives timing errors fed back by a re-sampler through baseband signal processing, and interpolates the timing error to the direction of reducing the errors between the digital signals outputted from the A/D converting section 101g which outputs the signal to the fixed passband matched filter 202b. 
The fixed passband matched filter 202b filters the signal, which has been symbol-synchronized and outputted by the symbol recovery section 201b, and readjusts the signal to maximize the SNR at the symbol position.
In theory, receiving filters have characteristics of a rectangular band, and cannot realize the shape of a receiving filter having an infinite time delay. Thus, the detecting procedure in a system having such characteristics is quite sensitive to even a slight timing error, and although an inter-symbol influence can be avoided at an accurate sampling time, inter-symbol interference occurs where a slight error exists. Accordingly, a slight excess bandwidth is required to realize the system. For that purpose, the matched filter 202b is generally used for the digital demodulation section 102.
The output of the matched filter 202b is inputted to the phase separation section 203b, and is separated to an I signal and a Q signal to be outputted to the complex number multiplying section 204b. The complex number multiplying section 204b receives restored and fed-back carrier waves COS and SIN, which have been multiplied by the I, Q passband digital signals of the phase separation section 203b, and transfers the I, Q passband digital signals to the complex multiplying section 204b where the I, Q baseband digital signals are fed back to the symbol recovery section 201b and outputted to the carrier wave recovery section 205b. 
The carrier wave recovery section 205b removes a frequency offset and phase jitter of the carrier wave from the I, Q baseband digital signals outputted from the complex multiplying section 204b, and feeds back the signals to the corresponding COS(ω) and SIN(ω) inputs of the complex number multiplying section 204b, while outputting the I, Q baseband digital signals with restored frequency offset and phase jitter to the channel equalizing section 206b. 
The channel equalizing section 206b removes the inter-symbol interference caused by multi-paths from the signals restored by the carrier wave recovery section 205b to be outputted to the channel decryption section 207b. Thus, in a digital transmission system such as an HDTV, a transmission signal causes a bit detection error at the receiver due to the signal distortion generated due to passing through a channel, interruption by an NTSC signal, or distortion caused by a transmission/receipt system. In particular, the radio wave of the signal traveling through multi-paths causes an inter-symbol interference, which becomes a major cause of bit detection errors. Accordingly, the channel equalizing section 206b removes the inter-symbol interference.
The channel decryption section 207b removes a burst jitter and a sporadic jitter existing in the channel that is contained in the signal, from which the inter-symbol interference has been removed by the channel equalizing section 206b to be outputted for recovery of the transmission symbol.
Thus, the conventional digital demodulators, which are embodied in three structures as shown in FIGS. 2a to 2c in accordance with the structural characteristics of a filter in the form of a matched filter, poses the following problems.
First, the digital demodulator shown in FIG. 2a that uses a fixed baseband matched filter of two channels and a re-sampler of two channels is compatible with the OFDM/QAM/QPSK demodulation method but incompatible with any of the VSB demodulation methods. To be specific, if a baseband matched filter is used, the VSB demodulation method having single side band (SSB) characteristics causes a frequency distortion around DC, as shown in FIGS. 3a and 3b, when demodulating a passband signal to a baseband signal. Moreover, circuit complexity drastically increases due to a requirement of the fixed baseband matched filter of two channels and the re-sampler of two channels.
In other words, FIG. 3a shows frequency characteristics of the VSB passband digital signal of FIG. 2b, while FIG. 3b shows frequency characteristics of a VSB baseband digital signal appearing after filtering the demodulated passband VSB digital signal by means of the fixed baseband matched filter. FIG. 3b shows frequency distortion characteristics around the DC. Such a frequency distortion deteriorates an SNR performance of the demodulator, and places a burden on the channel equalizing section.
The VSB digital demodulator using the fixed baseband matched filter of a single channel and the re-sampler of a single channel as shown in FIG. 2b is incompatible with all the VSB (OFDM)/QAM/QPSK demodulation methods. To be specific, since the passband digital signal, for which a carrier wave has not been recovered as shown in FIG. 4a, is used for an input of the fixed baseband matched filter, an output spectrum of the passband digital signal passed through the fixed baseband matched filter has distorted frequency characteristics corresponding to the carrier frequency offset. Such a spectrum distortion of the passband digital output signal causes an inter-symbol interruption, thereby resulting in an increase of the additive white Gaussian noise (AWGN), which is a major cause of bit detection errors as well as a deterioration of a bit error rate (BER) and a signal to noise ratio (SNR) as shown in FIG. 5. Therefore, a demodulator including a fixed passband matched filter is incompatible with any of the VSB (OFDM)/QAM/QPSK demodulation methods.
FIG. 4a shows frequency characteristics of input data having a frequency response of the fixed passband matched filter and a frequency offset (−1000 KHz). FIG. 4b shows frequency distortion characteristics of a passband digital signal appearing after filtering the input data having the frequency offset (−1000 khz) as shown in FIG. 4a by means of the fixed passband matched filter. Such a frequency distortion deteriorates the SNR performance of the demodulator, and places a burden on the channel equalizer. FIG. 5 is a graph showing experimental values of a declining curve of the SBR performance of the receiver due to the frequency distortion as shown in FIG. 4a drawn in accordance with the frequency offset of the fixed passband matched filter. In case of a 256-QAM demodulation method, approximately 7 dB of SNR deteriorates under the frequency offset of 300 KHz with respect to the frequency offset of 0 Hz. In case of the 8VSB demodulation method, approximately 4 dB of SNR deteriorates under the frequency offset of 300 KHz with respect to the frequency offset of 0 Hz. These numerical values were measured at the output terminal of the channel equalizing section.
The digital demodulator of FIG. 2c using the fixed passband matched filter of a single channel for the 8VSB demodulation method and the fixed baseband matched filter of two channels for the OFDM/QAM/QPSK demodulation method may be used to supplement the drawbacks of the digital demodulator as shown in FIGS. 2a and 2b but cannot eliminate the problems of deterioration in the SNR performance as well as the escalating circuit complexity subsequent to employing the fixed baseband matched filter of two channels, as shown in FIGS. 4a and 4b. 
In particular, the circuit complexity drastically escalates subsequent to deterioration of the SNR performance and the number of taps of the matched filter, as shown in FIG. 5. Thus, inconvenience still exists to determine what kind of matched filter should be employed depending on the demodulating method: the fixed passband matched filter 202c or the fixed baseband matched filter 205c. For instance, the fixed passband matched filter 202c is employed and the fixed baseband matched filter 205c is bypassed, when the demodulation method is 8VSB. The fixed baseband matched filter 205c is employed and the fixed passband matched filter 202c is bypassed when the demodulation method is OFDM/QAM/QPSK.